I have a Jannd that has served me well. It’s big enough that I can pack a lunch and a change of clothes with plenty of room left over, and the zipped pocket on the top is roomy enough to hold a rain jacket or cape. A U-lock might fit in there, but I’ve never tried it — one would certainly fit in the main compartment, or hung on the side with the Velcro loops. It has D-rings for a shoulder strap, which is a great thing to have. And it’s nice to have the optional extra capacity when it’s needed.

My Jannd does have thick padded walls, but they don’t make any claims about it being useful as a cooler (though they do say it will hold “an eight-pack of soda”). I see that Avenir does advertise their bag as having a fully-insulated main compartment, though.